Method for specifying and fabricating an object, associated apparatus, and applications

ABSTRACT

A method for specifying and fabricating non-homogeneous, anisotropic, truly functionally graded objects. The objects may have defined spatial heterogeneity (e.g., a gradient in material concentration) with local randomized distributions. This local randomness is designed in such a way that global averaging results in the specified spatial heterogeneity. The fabrication of structures is an additive process in which a material is deposited in defined patterns.

RELATED APPLICATION DATA

The instant application claims priority to U.S. provisional applicationSer. No. 61/448,710 filed on Mar. 3, 2011, the subject matter of whichis hereby incorporated by reference in its entirety.

BACKGROUND

Embodiments of the invention are generally directed to methods forspecifying (and optionally fabricating) an object to be fabricated; moreparticularly, to a method for specifying (and optionally fabricating) anobject to be fabricated by 3-D printing of arbitrarily definedfunctionally graded materials via solid freeform fabrication; and mostparticularly, to a method for specifying (and optionally fabricating)living tissue and other functionally graded, heterogeneous objectscharacterized by material heterogeneity (or blending) on a local scale.

Solid freeform fabrication (“SFF”) is the name given to a class ofmanufacturing methods that allow the fabrication of three-dimensionalstructures directly from computer-aided design (“CAD”) data. SFFprocesses are generally additive, in that material is selectivelydeposited to construct the object to be fabricated rather than removedfrom a block or billet. Most SFF processes are also layered, meaningthat a geometrical description of the object to be fabricated is cut bya set of parallel surfaces (planar or curved), and the intersections ofthe object and each surface, referred to as slices or layers, arefabricated sequentially. Together, these two properties mean that SFFprocesses are subject to very different constraints than traditionalmaterial removal-based manufacturing. Nearly arbitrary object geometriesare achievable, no tooling is required, mating parts and fully assembledmechanisms can be fabricated in a single step, and multiple materialscan be combined, allowing what has been referred to in the literature as‘functionally graded’ material properties. It is to be noted, however,that the term ‘functionally graded’ as used in the art refers to themacroscopic combination of materials in a layer, akin to pieces of ajig-saw puzzle, but not to a localized, microscopic-scale heterogeneityor blending of materials. Notwithstanding, new features, parts, and evenassembled components can be “grown” directly on already completedobjects, suggesting the possibility of using SFF for the repair andphysical adaptation of existing objects. On the other hand, a depositionprocess must be developed and tuned for each material, geometry islimited by the ability of the deposited material to support itself, bythe (often poor) resolution and accuracy of the process, and multiplematerial and process interactions must be understood.

SFF has traditionally focused on printing passive mechanical parts orproducts in a single material, and research has emphasized developingnew deposition processes, improving quality, resolution, and surfacefinish of fabricated objects, and broadening the range of singlematerials that can be employed by a given SFF process, includingbiocompatible polymers and other biomaterials and living cells. Theseimprovements have allowed freeform fabrication to become a viable meansof manufacturing finished functional parts, rather than only prototypes.

More recently, the greater utility of freeform fabricated objects havingmultiple materials has been recognized, prompting reexamination andnovel research into processes which can fabricate objects using multiplematerials and, which can thereby produce complex articles with a varietyof functionality, including functionally graded materials. All of thesesystems still depend upon a small fixed set of deposition processtechnologies, and are therefore limited to the materials that can beadapted to those processes, by the effects of those particular processeson the materials, and by the fabrication rates and resolutions of thoseprocesses. For example, the system of U.S. Pat. No. 6,905,738 toRingeisen et al. requires that for every material to be deposited, a twomaterial system be developed comprising the material to be transferred,and a compatible matrix material that is vaporized by the laser in orderto propel the transfer material to the substrate. In addition, thissystem has only demonstrated fabrication of thin films of materials; itsability to deposit many layers of materials is not well established. Thesystem and method of Sun et al., “Multinozzle Biopolymer Deposition forTissue Engineering Application,” 6.sup.th International Conference onTissue Engineering, Orlando, Fla. (Oct. 10-13, 2003) and InternationalPatent Application No. PCT/US2004/015316 to Sun et al., is limited to afixed set of four deposition processes and requires that alginatematerials be deposited into a bath of liquid cross linking agent, alimitation it shares with the work of Pfister et al., “Bio functionalRapid Prototyping for Tissue-engineering Applications: 3D BioplottingVersus 3D Printing,” Journal of Polymer Science Part A: PolymerChemistry 42:624-638 (2004) and Landers et al., “Desktop Manufacturingof Complex Objects, Prototypes and Biomedical Scaffolds by Means ofComputer-assisted Design Combined with Computer-guided 3D Plotting ofPolymers and Reactive Oligomers,” Macromolecular Materials andEngineering 282:17-21 (2000). In addition, none of these systemsexplicitly measures the properties of, and monitors and controls theconditions experienced by the fabrication materials, the fabricationsubstrate, and the article under construction before, during, and/orafter fabrication as an intrinsic part of the fabrication process andmanufacturing plan. The fabrication process is thus limited to thespatial control of material placement on relatively simple, passivesubstrates. Temporal control of the evolution of material properties istherefore not possible, and complex substrates whose state must becontrolled and monitored continuously are not readily accommodated.Fabricating into or onto substrates, such as living organisms or deviceswhich must remain in operation continuously, is problematic.

A major challenge in orthopedic tissue engineering is the generation ofseeded implants with structures that mimic native tissue, both in termsof anatomic geometries and intra-tissue cell distributions. Previousstudies have demonstrated that techniques such as injection molding(Chang et al., “Injection Molding of Chondrocyte/Alginate Constructs inthe Shape of Facial Implants,” J. Biomed. Mat. Res. 55:503-511 (2001))and casting (Hung et al., “Anatomically Shaped Osteochondral Constructsfor Articular Cartilage Repair,” J. Biomech. 36:1853-1864 (2003)) ofhydrogels can generate cartilage tissue in complex geometries. Otherstudies have investigated methods to reproduce regional variations inarticular cartilage constructs by depositing multiple layers ofchondrocytes (Klein et al., “Tissue Engineering of Stratified ArticularCartilage from Chondrocyte Subpopulations,” Osteoarthritis Cartilage11:595-602 (2003)) or chondrocyte-seeded gels (Kim et al., “ExperimentalModel for Cartilage Tissue Engineering to Regenerate the ZonalOrganization of Articular Cartilage,” Osteoarthritis Cartilage11:653-664 (2003)). However, there remains no viable strategy forrapidly producing implants with correct anatomic geometries and celldistributions. Recently, advances in SFF techniques have enabled thedeposition of multilayered structures composed of multiple chemicallyactive materials (Malone et al., “Freeform Fabrication of 3D Zinc-AirBatteries and Functional Electro-Mechanical Assemblies,” RapidPrototyping Journal 10:58-69 (2004)).

Tissue failure is a serious condition that can lead to deterioration oflifestyle and potentially to death. Medical advances in the past severaldecades have enabled the replacement of damaged tissues with mechanicalor biochemical implants designed to mimic the function of the defectivetissues. While these implants, such as, e.g., heart valves and hip-bonereplacements, have demonstrated promising performance, such implants aresusceptible to wear and degradation over time. More critically, theseimplants are incapable of repair or regeneration. This can becomefatally problematic for infants and younger patients since their bodiescan outgrow the implants and subsequently cause implant failure.Multiple surgeries would be necessary for implant resizing, but suchoperations can be traumatic for patients. While some conditions can betreated with implants, other failures in larger tissues, such as theheart and liver, are more serious since implants that can replace thesetissues do not exist. As a result, patients rely on donors fortreatment, but the demand of such organs far exceeds the availablesupply.

Tissue engineering has the potential to alleviate these limitations byproviding living tissue replacements capable of growth and integration.A major technical challenge facing the field as a whole, however, is toboth effectively fabricate tissue geometry and to specify (and/orcontrol) locally heterogeneous tissue biomechanics. Creating complex,non-symmetric geometries that are applicable to tissue engineering canbe approached using solid-freeform fabrication (SFF), orthree-dimensional (3D) printing. Geometrically complex hard tissuesincluding knee meniscus, bone, and cartilage can be fabricated. 3-Dprinting of highly extensible soft tissues, meanwhile, can be morechallenging because gravity forces cause deposited viscous fluids tospread unless they are crosslinked, which limits the building of tall ornon-self supportive geometries such as heart valves.

Pertinent object fabrication systems and methods are disclosed in U.S.Pat. Nos. 7,625,198, 7,939,003, and Pub. No. US 2011/0169193, thesubject matters of which are hereby incorporated by reference in theirentireties. To date, however, materials are deposited as separate,homogeneous materials, not as an interspersed or blended heterogeneouscomposite as would be required, for example, for truly functionallygraded tissue engineering and fabrication. (i.e., composites withgradual variation in composition and structure, creating differences inmaterial properties over the volume as, for example, by specifyingdistribution gradients as functions of spatial position).

The inventors have thus recognized the benefits and advantages providedby solutions to the aforementioned problems and known challenges in thisfield. These solutions include, among other things, the ability tosimultaneously solid freeform fabricate and crosslink anatomicallyprecise constructs in the form of stable, soft tissue structures; theability to specify and fabricate tissues exhibiting localizedheterogeneous (blended) biomechanics; the ability to access the printaccuracy of the constructs.

SUMMARY

An embodiment of the invention is a machine-implemented method forspecifying a target object to be fabricated. The method includes thesteps of i) defining the target object to be fabricated; ii) determininga locally-heterogeneous, anisotropic distribution of phenotypiccharacteristics of the target object to be fabricated; iii) ascribingthe distribution of phenotypic characteristics to a plurality ofmaterials from which the target object will be fabricated; iv) stitching(i.e., joining or otherwise combining) together each distributioncorresponding to respective ones of the materials into respective setsof vector paths corresponding to respective ones of the materials,wherein each set of vector paths is one or more line elements of a givenlength and orientation; and v) creating a vector print file for thetarget object readable by a compatible fabrication machine such as,e.g., a software-controlled 3-D printer. Note that the created vectorpaths need not be limited to straight lines or Cartesian coordinates(e.g., curves can be approximated as many short lines to make thetraversing motor of the fabrication apparatus move in the appropriatedirections). In various exemplary, non-limiting aspects, the methodfurther includes the following steps, processes, and/or characteristics:

-   -   wherein the step of defining the target object to be fabricated        comprises either sourcing the target object from a prescribed        image or from a mathematical model;        -   wherein the step of sourcing the target object from a            prescribed image further comprises reconstructing the image            of the target object into a 3-D machine-readable format;            -   wherein the step of sourcing the target object from a                mathematical model further comprises defining the 3-D                geometry of a selected target object;            -   determining a gradient distribution of material                compositions of the target object to be applied to the                geometry;            -   establishing a probability distribution function based                on the gradient distribution to determine a plurality of                path segments and assigning a plurality of one or more                of the path segments to a corresponding plurality of                materials from which the target object will be                fabricated;            -   dithering the pluralities of path segments based on the                probability distribution function;            -   converting the dithered path segments into respective                sets of vector paths corresponding to respective ones of                the materials, wherein each set of vector paths is one                or more line elements of a given length and orientation;                and            -   creating a vector print file for the target object                readable by a compatible fabrication apparatus;                -   wherein the gradient distribution is linear;                -   wherein the gradient distribution is non-linear;                -   wherein the gradient distribution is radial;                -    wherein the gradient distributions are                    representative of planar and volumetric imaging data                    (e.g., computed tomography (CT));                -   further comprising outputting the vector print file                    for the target object to the compatible fabrication                    apparatus and fabricating the target object.                -    further comprising fabricating the target object in                    a series of n fabrication layers, and curing a                    selected region of each n^(th) fabrication layer                    simultaneously with the layer fabrication or prior                    to the fabrication of the n^(th)+1 layer.            -   the method further comprising obtaining a plurality of                image layers; determining a desired image resolution;                selecting one or more of the plurality of image layers                as an n^(th) fabrication layer image, where n is equal                to or greater than one; and optionally, resizing the                n^(th) fabrication layer image for specifying at least                one of a size and a volume characteristic of the target                object;                -   wherein the step of determining a                    locally-heterogeneous, anisotropic distribution of                    phenotypic characteristics of the target object to                    be fabricated further comprises assigning values                    between zero to one to a each of a plurality of                    pixels or voxels, or to each of a plurality of sets                    of adjoining pixels or voxels, of the n^(th)                    fabrication layer image;                -    wherein the step of ascribing the distribution of                    phenotypic characteristics to a plurality of                    materials from which the target object will be                    fabricated further comprises associating a plurality                    of the assigned values to respective ones of the                    materials;                -    wherein the step of associating a plurality of the                    assigned values to respective ones of the materials                    further comprises thresholding the assigned values;                -    wherein the step of associating a plurality of the                    assigned values to respective ones of the materials                    further comprises determining Hounsfield unit values                    for the pixels or voxels;                -    further comprising removing background that is not                    a part of the target object from the nth fabrication                    layer image;                -    further comprising creating temporary support                    structures to aid in the stability of subsequently                    fabricated layers of the object;                -    wherein the step of stitching together each                    distribution further comprises dithering each of the                    plurality of the assigned values into a plurality of                    segments corresponding to respective ones of the                    materials;                -    further comprising utilizing a Floyd-Steinberg                    dithering algorithm or other known dithering                    algorithm;                -    further comprising converting each of the plurality                    of segments into the vector paths for the respective                    ones of the materials;                -    wherein the step of creating a vector path further                    comprises a) selecting a starting index; b)                    comparing the starting index to an adjacent index in                    a specified direction; c) determining whether the                    starting and adjacent indices are assigned to the                    same one of the materials, and d) if the starting                    and adjacent indices are assigned to the same one of                    the materials, then (e) creating a vector path                    between the indices, or (f) if the starting and                    adjacent indices are not assigned to the same one of                    the materials, then g) changing the adjacent index                    into a new starting index and performing steps                    (b-e); and h) performing steps (a-g) until a                    sufficient number of vector paths have been                    generated to specify the target object;                -    further comprising iteratively performing steps                    (iii-v) for a plurality of the n^(th) fabrication                    layers, where n is equal to or greater than one;        -   further comprising iteratively performing steps (iii-v) for            a plurality of n fabrication layers, where n is equal to or            greater than one;    -   wherein the phenotypic characteristics of the target object to        be fabricated are surface and/or internal phenotypic        characteristics;    -   further comprising outputting the vector print file for the        target object to the compatible fabrication apparatus and        fabricating the target object;        -   further comprising outputting the vector print file for the            target object to the compatible fabrication apparatus and            fabricating the target object;            -   further comprising fabricating the target object in a                series of n fabrication layers, and photo-curing a                selected region of each n^(th) fabrication layer                simultaneously with the layer fabrication or prior to                the fabrication of the n^(th)+1 layer;        -   further comprising fabricating the target object in a series            of n fabrication layers, and photo-curing a selected region            of each n^(th) fabrication layer simultaneously with the            layer fabrication or prior to the fabrication of the            n^(th)+1 layer.

Embodiments of the invention enable solid freeform fabrication (e.g.,3-D printing) of objects using arbitrarily defined functionally gradedmaterials. Functionally graded materials are composites with gradualvariation in composition and structure, creating differences in materialproperties over the volume of the object. A defined, functionally gradedmaterial can be specified, and fabricated, by specifying distributiongradients as, e.g., functions of spatial position, in an automated,computer controlled process (algorithm). The algorithm can generatedeposition paths that are communicated to a fabrication robot forproduction of the object from the functionally graded materials.

In an exemplary embodiment, the invention enables fabrication of afunctionally graded object from cell-seeded photocrosslinkable hydrogelswith differing mechanical properties and differing cell populations fortissue engineering. The gradients in composition and structure can bedefined to reflect native tissue, serving as a basis for accuratescaffold production for, e.g., research or therapeutics. Gels areselectively deposited as viscous liquids and then crosslinked byexposure to UV to produce a three-dimensional solid structure.

Curing or crosslinking of the hydrogels via UV exposure can beaccomplished using a software-controlled adjustable-beam UV source. Anadjustable or interchangeable lens system permits focusing of the beamto change the illumination intensity and spot size for small pointcuring or broad illumination. A bipolar junction transistor can serve asa switch that permits the passing of current to the UV source when an ONcommand is issued by software, and fails safe into an OFF position. Foradditional controllability, a potentiometer or amplifying element can beused to adjust the circuit, thereby tuning the intensity of the UVsource.

Non-photocrosslinkable support structures may also be implemented.Support structures may advantageously limits a current layer fromsettling and spreading out before it can be cured; otherwise, the layermay be shorter than expected and cause the fabrication (e.g., extrusion)of the next layer to miss the object's upper surface. Support structuresmay also help to make the object's upper surface flat for subsequentsurfaces to print on, thereby limiting compound errors.

This technology is advantageous for a number of material scienceapplications requiring precise control over three-dimensional shape andintegrated multi-material composition. Previous technology focused onthe deposition of functionally separate but adjoining entities, whichdoes not represent heterogeneity on a local scale. The instanttechnology also enables the acquisition of local material geometry andmultiaxial material gradients using data extractable from a variety ofimaging platforms. This technology is fundamental to a number ofimportant freeform fabrication applications, including but not limitedto: automatic fabrication of materials that have locally spatiallyvarying material characteristics that can be prescribed directly,randomized, or created according to a distribution function comprisingtwo or more components along two or more directions; improved sealingmethod to meld multiple distinct materials in 3D space, whether as oneor multiple connected objects; for biological applications in which thistechnology enables the creation of materials that recapitulate intrinsiclocal material heterogeneity, and when combined with cells enablesfabrication of living heterogeneous materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A-D) show examples of fabricating a heart valve replacementaccording to an exemplary embodiment of the invention;

FIG. 2 shows the results of printed heart valve samples printed withsingle material (8000 MW PEG-DA) using the data obtained from FIG. 1,according to an illustrative aspect of the invention;

FIGS. 3(A-D) show an illustrative aspect that allows the crosslinkingtime of hydrogels to be controlled in several ways, according toillustrative aspects of the invention;

FIGS. 4A and 4B show LED crosslinking modules located closer to, andfurther from the hydrogel surface, respectively; FIG. 4C shows a switchthat can be used to turn the crosslinking module on and off, accordingto illustrative aspects of the invention;

FIGS. 5(A-C) illustrate a fabricated heart valve model and process,according to illustrative aspects of the invention;

FIG. 6 illustrates other exemplary geometries printed withphotocrosslinking hydrogels using the embodied crosslinking module,according to illustrative aspects of the invention;

FIG. 7A shows the smallest commercially available mechanical heartvalve; FIG. 7B shows printed valves in different sizes, demonstratingthat valves can be fabricated to sizes specific to a patient, accordingto illustrative aspects of the invention;

FIGS. 8(A-C) illustrate an assessment of the embodied printing method,according to illustrative aspects of the invention;

FIGS. 9(A-D) illustrate the fabrication method and apparatus forprinting living tissue, according to illustrative aspects of theinvention;

FIG. 10 shows an (x, y, z) coordinate table (left) and an illustrationof a two-dimensional vector print layer (right), according to anillustrative embodiment of the invention;

FIGS. 11A, B, respectively, illustrate the generation of linear andcircular gradients, according to illustrative aspects of the invention;

FIG. 12 is a flowchart describing the programmable machine-enabledprocess steps for specifying (and ultimately fabricating) geometriesexhibiting anisotropic heterogeneity based on prescribed images,according to an embodiment of the invention;

FIG. 13 is a flowchart that further expands the step of convertingdithered paths into vector paths, according to an embodiment of theinvention;

FIG. 14 is a schematic block diagram illustrating the process ofcreating vector paths between dithered segments, according to anillustrative aspect of the invention;

FIG. 15 is a schematic block diagram illustrating a non-ideal scenarioof converting dithered segments into vectors due to lack ofdirectionality, according to an illustrative aspect of the invention;

FIGS. 16( a-d) are schematic block diagrams illustrating an optimized,prioritization scenario of converting dithered segments into vectors bydirectionality, according to an aspect of the invention;

FIG. 17 is a flowchart describing the programmable machine-enabledprocess steps for specifying (and ultimately fabricating) geometriesexhibiting anisotropic heterogeneity based on mathematical models thatare different than the process steps shown in FIG. 12, according to anembodiment of the invention;

FIG. 18 illustrates a printable layer exhibiting a locally heterogeneouslinear gradient based on a linear probability function, according to anillustrative aspect of the invention;

FIG. 19 illustrates a printable layer exhibiting a locally heterogeneousgradient in two directions (diagonal) based on a two dimensionalprobability function, according to an illustrative aspect of theinvention;

FIG. 20 illustrates a printable layer exhibiting a locally heterogeneouscircumferential gradient based on a radial probability function,according to an illustrative aspect of the invention; and

FIG. 21 shows an example of a heterogeneous geometry printed withmultiple hydrogel blends.

DETAILED DESCRIPTION OF EXEMPLARY, NON-LIMITING EMBODIMENTS

According to an aspect, the fabrication method described herein is anadditive process that rapidly recreates 3-D computer-aided design (CAD)models. This is accomplished by slicing the models into layers andbuilding the layers upward using deposition tools that can extrudedifferent materials. Materials that can be used to build tissueengineered constructs include hydrogels, which may be photo-cured(crosslinked) or otherwise structurally solidified. Hydrogels aregenerally aqueous and are attractive fabrication materials since theycan be freely extruded into desired shapes. In addition, hydrogels formlong-term, robust structures upon photocuring by radiation exposure(e.g., UV).

An exemplary printing method uses PEG-DA (poly(ethyleneglycol)-diacrylate hydrogels, which polymerize when exposed to UVradiation at 365 nm. However, the application of this process can beextended to other photocrosslinking hydrogels such as, but not limitedto, PEG-DA-PEA (poly-ester amide), GMA-chitosan, and alginate.

FIGS. 1(A-D) show an example of fabricating a heart valve replacementusing the embodied invention. Starting at FIG. 1A, a native porcineaortic valve was scanned using micro-computer tomography (micro-CT). Thetissue exhibits the three leaflets that direct the blood to flowunidirectional. The surrounding wall, or the root, joins and stabilizesthe leaflets together. The scanned valve, which can be visualized in thewhitish (lighter) regions (FIG. 1B), is then reconstructed into a 3-Dstereolithography file (STL; FIG. 1C), which is a CAD-based format. Oncethe STL models are imported into the printing system, the softwareslices the model into layers. The deposition then prints the contour (inred) and the inner regions of each layer (green) (FIG. 1D). FIG. 2 showsthe results of printed heart valve samples printed with a singlematerial (8000 MW PEG-DA) using the data obtained from FIG. 1.

In an aspect, printing and crosslinking of hydrogels is an alternatingprocedure: after printing a layer, the printing is paused and UV lightis exposed onto the deposited layer to crosslink the hydrogels. Thisprocess is repeated until the construct is built. In another aspect,structures such as those made from hydrogels can also be accurately andefficiently printed and crosslinked simultaneously. The UV source usedto crosslink must provide sufficient power to initiate the crosslinkingreaction. A crosslinking module containing, e.g., UV-LEDs can be mountedparallel to the surface, and directly below the deposition tools.Placing the module this way can provide enhanced UV exposure to thehydrogels, resulting in more efficient hydrogel crosslinking duringprinting. It should be appreciated that photocuring and, particularlyphotocuring with UV, is exemplary; however, any applicable chemical orother energy source useful for solidifying or structurally stabilizing amaterial(s) during object fabrication is envisioned herein. Appropriatecuring can be performed on the n^(th), n^(th)+1, and/or n^(th)−1 layers.

To build hydrogel structures, controlling the timing of the hydrogelcrosslinking is important for successful fabrication. If the hydrogelscrosslink too slowly, the construct will be unstable because eachprinted layer does not crosslink enough to provide structural supportfor subsequent layers. Hydrogels crosslinking too fast is alsoproblematic since each layer can crosslink completely before thefollowing layer is printed. This prevents different layers from fusingwith each other, which makes the constructs susceptible to delamination.

FIG. 3A shows an illustrative design aspect that allows the crosslinkingtime of hydrogels to be controlled in several ways. First, the poweroutput of the crosslinking module can be adjusted by changing theelectrical resistance through a potentiometer (FIG. 3B). Reducing theresistance increases current through the LEDs (and subsequently thepower output) (FIG. 3C). This can accommodate the necessary current tocrosslink different materials, which could have different crosslinkingbehavior (i.e. crosslinking time, color). For example, 8000 MW PEG-DAcrosslinks slower than 700 MW PEG-DA when exposed to the same amount ofUV radiation (FIG. 3D). One would thus increase the power when printingthe 8000 MW PEG-DA hydrogel so that the crosslinking time becomescomparable to that of 700 MW PEG-DA. The LEDs on the module can also beplaced at different locations in the circuit as illustrated in FIG. 4:A) closer to surface; B) further from surface). Locating hydrogelscloser to the surface leads to faster crosslinking, and locating themfurther away would lead to slower crosslinking. The crosslinking modulemay also be turned on and off using a switch (FIG. 4C). This controlallows the LEDs to be turned off momentarily to prevent overcrosslinkingof gels. The switch can be operated manually or can be enabled by theprinter.

Using the above described or other suitable crosslinking module,geometries can be printed more accurately. Starting with the valve scanshown in FIG. 1A, the valve leaflets were separated from the surroundingroot via thresholding. The leaflets have a less dense tissue densitythan the root. As a result, the leaflets appear whiter than thesurrounding root. Based on the grayscale pixel values, the leafletregions can be thresholded and isolated from the root (FIG. 5A). Thefinished STL model after cleanup is shown in FIG. 5B (leaflet: red;root: blue). The models shown in FIG. 5 were printed with two differentmaterials and photocured using the UV crosslinking system describedhereinabove. The similarity between the STL model (FIG. 5B) and theprinted scaffold (FIG. 5C) is evident. The printed valve now alsofeatures the coronary ostium, a tube that protrudes from the root tochannel blood directly to the heart itself.

Other illustrative geometries can also be printed with photocrosslinkinghydrogels using the above described crosslinking module, as shown inFIG. 6. Note however, the embodied object specification and fabricationmethod embodiments disclosed herein are not limited to a particular typeof medium, method of material deposition, method of solidification(e.g., curing), or object specified or fabricated. Rather, the inventionpertains to specifying and generating vector-based deposition paths fora selected material(s) to specify and, optionally fabricate virtuallyany object of interest. Although extruding the material(s) is disclosedas an advantageous aspect of the invention, other techniques may includesintering material(s) in these paths, laying molten metal in these paths

FIG. 7B shows printed valves in different sizes, demonstrating thatvalves can be fabricated to sizes specific to a patient. The largestvalve spanned 22 mm (adult size) in inner diameter while the smaller onespanned 12 mm (children size). In comparison, the smallest commerciallyavailable mechanical valve spans 17 mm (FIG. 7A), a size too large forchildren.

In addition to building the constructs in the right scale, implants alsoneed to be printed accurately to fit into the patient. To access theaccuracy of the printing method, we analyzed the shape fidelity of theprinted constructs. Printed constructs first underwent CT scan, and theresulting images were matched to the target print images (FIG. 1D). Byoverlapping the scanned and target images, we can identify the regionsprinted accurately (overlap; FIG. 8A). The comparison map also indicatesregions where too much material (overprint) and too little material(underprint) were printed. The largest valve was printed most accurately(89%) while the smallest was printed least accurately (73%); (FIG. 8B).Analysis also showed that soaking the valves in buffer solution afterprint causes the scaffold to swell, causing the geometry to changethrough expansion. As a result, swelling decreased the print accuracy onfully hydrated samples, but only less by 1-2% to correspondingnon-saturated samples (FIG. 8C).

To demonstrate that the embodied fabrication technique can yield livingtissues, we generated hydrogel valve constructs with cells attached andcultured in the scaffold (FIGS. 9(A-D)). Initially a sterile tent wasbuilt using an autoclave bag, although the tent may be composed of anysterile material that could encompass the hydrogel. After punching holesonto the tent, the tent was sealed and autoclaved. The tent was thenunsealed in a sterile culture hood, and the deposition tools printedhydrogels inside the tent through the holes. The crosslinking moduleattached to the deposition tool transmitted UV light through the tent tocrosslink hydrogels.

After valves were printed and subsequently rinsed in buffer, they wereplaced in vials containing media and valve interstitial cells (VICs).Vials were spun for 24 hrs so that cells were attached onto the valve.Cell-seeded valves were then cultured in culture flasks filled withmedia for days 1, 7, and 21 at 37 C.° and 5% CO₂. After culture, theroot and leaflet sections were imaged to observe cell viability. In allcultured samples, dead cells were not detected, suggesting 100%viability for all days.

Although valve geometries replicating the native structure can begenerated as demonstrated above, the greater challenge in tissueengineering is to replicate the heterogeneous biomechanics and the cellphenotype inherent in the native tissue. Merely printing the valve rootwith stiffer materials and the leaflet with more compliant materials isinsufficient because this process over-simplifies the valvebiomechanics. In the native valve, for example, the leaflets arecomposed of three distinct regions: fibrosa, spongiosa, andventricularis. The fibrosa is concentrated with collagen, which helpsprovide strength and stiffness to the leaflets; the ventricularis iscomposed of elastin, which helps the valve return to original shapeafter stretching; the spongiosa, located between the two layers, isconcentrated with glycosaminoglycan, sugar molecules that act assupporting cushions between the fibrosa and ventricularis. As a result,some regions in the leaflet will be stiffer than others, even though theleaflet is overall a compliant tissue.

If, for instance, a region A is stiffer while a region B is morecompliant, we would expect the region between A and B to exhibitintermediate stiffness, since a mechanical gradient from A to B exists.Investigating how to recreate these gradients is critical for tissueengineering studies. For instance, the gradient can cue progenitor cellsseeded on the tissue to differentiate into various phenotypes based onthe mechanical properties of the construct. Additionally, such modelscan help in the study of diseases, such as valve calcification, thatoccur notably on the interface regions.

To accomplish this task, we utilize a process referred to herein as‘vector printing’ that enables true locally heterogeneous,functionally-graded fabrication via 3-D printing, as illustrated in FIG.10. A vector printing module commands the 3-D printer to print from onevertex to another. The 3-D printing system allows the user to inputinstructions for defining print paths in Cartesian coordinates. For asingle layer, material is deposited in an x-y plane. As shown in FIG. 10(right), two vectors are in a single layer. The printer reads thecoordinate points (x,y,z) from a file and extrudes material along thatvector/path (square symbols). A new extrusion path can be specified in adifferent material or the same material (triangle symbols). Fabricatingsubsequent layers stacked in the z-direction yields a 3-D geometry. Thethickness of each layer is typically controlled by printing each layerat a specified interval of height. Unlike STL printing, vector printingprints by lines rather than by drawing and filling the outline of ashape. In the vector format, the start and end vertices are grouped witheach other. The printing platform will observe this pair and printaccordingly. An advantage of vector printing is that it can moredirectly specify the path to print than STL printing.

Briefly (a more detailed description is presented below), a datasetconsisting of sequential coordinates in a −D Cartesian space is defined.These spatial coordinates describe beginning and end points of pathsegments for material deposition. An exemplary method to transform thisdataset from simple vertices to defined path segments comprises thefollowing steps:

Vertices are defined as an ordered triplet (x, y, z) in Cartesian space.Paths for a fabrication robot are a sequence of spatial coordinates thatthe deposition tool traverses through in order. The end of a path isdenoted by a blank line. To create path segments, we use the initialdata set of vertices V_n and convert into a set of segments defined asS_n=[b_n; e_n; ‘\n’] where b_n=V_n−1 and e_n=V_n and ‘\n’ is thestandard end of line character. This creates a dataset defining shortpath segments or travel vectors S_n that begin at spatial coordinate b_nand end at coordinate e_n. An additional post-processing technique maythen be implemented to remove unwanted artifacts.

Two exemplary approaches for separation of these path segments fordeposition of different materials are based on the original vertex dataset; i.e., for the specification and ultimate fabrication of a definedtarget object originating from a) a mathematical model of a perceived(arbitrary) target object or, b) for a defined target object originatingfrom an image file of the object, e.g., Digital Imaging andCommunications in Medicine standard (DICOM), MATLAB, PNG, CT, or otherimage types. For the former origination source (a), a probabilitydistribution function can proscribe a spatial gradient of materialcompositions. As a simple example (without loss of generality) considera simple linear gradient. To define a gradual shift from a givenMaterial 1 to a given Material 2 along the x axis, we can define aprobability distribution function

P _(—)1(x)=1−x/X

where x is the mean x value of a path segment and X is the maximum valuein the x direction. To create the gradual shift in composition, a randomnumber is generated and compared with the value of P_(—)1(x) for eachpath segment. If the random number exceeds P_(—)1, the segment isassigned to be fabricated using Material 2, and if the random number isless than P_(—)1, the segment is fabricated by Material 1. Thedistribution is locally random but overall gradually shifts from 100%Material 1 to 100% Material 2 in a linear fashion. This concept can beextended to any probability density function (with dependence on any orall spatial directions) and is not restricted to binary distribution.For example, the embodied method enables arbitrary gradients to beestablished by generating images in multiple directions. FIG. 10Aillustrates a sequential linear gradient; FIG. 10B illustrates acircular gradient. Stacking the circular gradient would form the lighter(peach) regions into a cone.

Alternatively, for an image-originated object such as, e.g., a DICOMset, the method can use Hounsfield units or gray values to guide theassignment of materials. In the simplest case, we may assign a thresholdvalue; a pixel with intensity greater than the threshold may befabricated with Material 1, and otherwise it may be fabricated withMaterial 2. More sophisticated algorithms could, e.g., average theHounsfield/gray values of surrounding pixels to determine the materialdeposited at this spatial coordinate.

This concept is important to the specification (and fabrication) ofstructures that approximate native tissue. Computed tomography andproton density MRI scans (among others) encode gray values/Hounsfieldunits that are directly proportional to the density of the imagedtissues. This data is seldom used quantitatively, but by applying anappropriate transformation to the data, as embodied herein, it can beused to guide the selection of printing material. Note that virtuallyany parameter that can discriminate between different entities within animage, e.g., contrast, is useful however it is measured. In MRI andultrasound, for example, different methods of generating and measuringcontrast will be used, but will identify the same fundamentalcharacteristic of the tissue.

An exemplary aspect of this approach is for selection of aphotocrosslinkable hydrogel. Multiple hydrogels with distinct curingbehaviors could be selected from based on the Hounsfield values. Eachgel could be cured to varying degrees due to UV exposure time, and eachgel mixture could encompass a different range of stiffness that isdescribed by the Hounsfield values. Hence, a gel might be wellcharacterized to cure to a stiffness corresponding to Hounsfield valuesin the range [230-420], while another might fit the range [420-700]. Apixel with value 530 could then be produced by depositing a secondmaterial, which could be cured for a given length of time to achieve theappropriate mechanical properties.

Printing gradients with the desired line qualities is important forsuccessful fabrication of living tissue. Lines printed too thinly willresult in gaps between lines; lines printed too thickly will cause gelsto push and overlap with each other, creating bumps and uneven surfaces.To print with the desired line resolution, we first observe howadjusting the deposition rate changes the line thickness of the printedhydrogels. After printing a set of lines, the printed regions arethresholded. The pixels across the printed line are measured todetermine line thickness. This model aids in printing with theappropriate parameters so that the layers are free of gaps and bumps.

Another exemplary distribution approach embodied herein is a GraphicalUser Interface (GUI) that permits easy editing by a user who may need tocorrect a problematic error or introduce simple changes. This could beimplemented as an array of toggle buttons, check boxes, or color codedboxes, for example, each corresponding to a print path. The toggles(labeled by color, numerically, or symbolically, for instance) wouldallow distribution of a particular path to being printed by one of anynumber of materials at a click of a mouse. For extensive editing, aclick and drag functionality could be implemented.

In addition to quantifying line thickness, the spreading characteristicsof aqueous materials can be quantified as well. Spreading of aqueousmaterials such as hydrogels may occur after printing due to untimelycrosslinking. As a result, the line thickness of printed material canchange over time, and may cause the overall printed geometry to bedifferent from the intended shape. Predicting the spreading behaviorprovides the ability to make necessary adjustments to compensate forspreading. A camera can be used to observe and characterize thespreading progress in real time. To capture the spreading successfully,the camera can be in a fixed location over the printed material. In anillustrative aspect, a transparent glass was raised above the printstage and the camera placed directly below the glass; hydrogels werethen directly printed on the glass. Since the printed materials can bemade visible through the glass panel, the camera can record how theprinted line changes over time.

Heterogeneous Anatomical/Geometry Printing As described herein, aspectsof the method embodiment enable the specification and fabrication ofobject geometries exhibiting locally heterogeneous gradients.

Specifying and Fabricating Geometries Exhibiting AnisotropicHeterogeneity Based on Prescribed Images

FIG. 12 is a flowchart 1000 describing the programmable machine-enabledprocess steps for specifying (and optionally, fabricating) geometriesexhibiting anisotropic heterogeneity based on prescribed images. Theprocess begins at step 1010 by defining the target object to befabricated from a prescribed image file such as DICOM, MATLAB, oranother known file source. The image is then converted into theappropriate machine-readable format.

At step 1020, images of interest are selected and processed. Thisincludes determining a locally-heterogeneous, anisotropic distributionof phenotypic characteristics of the target object to be fabricated and,more particularly includes the steps of obtaining a plurality of imagelayers; determining a desired image resolution; selecting one or more ofthe plurality of image layers as an n^(th) fabrication layer image,where n is equal to or greater than one; and optionally, resizing then^(th) fabrication layer image for specifying at least one of a size anda volume characteristic of the target object.

At step 1030, the processed images are thresholded. This involvesascribing the distribution of phenotypic characteristics to a pluralityof materials from which the target object will be fabricated. Thisincludes the further steps of assigning values between zero to one toeach of a plurality of pixels or voxels, or to each of a plurality ofsets of adjoining pixels or voxels, of the n^(th) fabrication layerimage, and associating a plurality of the assigned values to respectiveones of the materials. This may further comprise determining Hounsfieldunit values for the pixels or voxels.

Step 1040 involves dithering and creating the print vectors, includingstitching together each distribution corresponding to respective ones ofthe materials into respective sets of vector paths corresponding torespective ones of the materials, wherein each set of vector paths isone or more line elements of a given length and orientation; andcreating a vector print file for the target object readable by acompatible fabrication apparatus. A Floyd-Steinberg dithering algorithmcan be used, for example. As used herein, the term ‘dithering’ refers tographical dithering generally defined as a technique involving the localintroduction of noise to acceptably represent a spectrum of color with alimited set of discrete colors while minimizing artifacts such as colorbanding. In our case, dithering ‘bins’ the full spectrum of materialproperties (or image contrast, however defined) into discrete materialsfor fabrication.

FIG. 13 is a flowchart 2000 that further expands the step in 1040 ofconverting dithered paths into vector paths. With reference thereto andas further illustrated in FIG. 14, once images are dithered intosegments that would be printed using different materials (e.g.,hydrogels), the software creates vector paths for fabricating the targetmodel. A starting segment is first selected and then compared to theneighboring segment. If the segments are assigned to the same material,a vector between the segments is created. This process repeats untilsufficient numbers of vector paths have been generated for fabricatingthe target geometry.

When comparing the starting segment to the adjacent segment, if nodirectionality is specified the resulting vector paths can exhibitnumerous directional changes. This requires more movement of the printertools, which can lead to inefficient and inaccurate fabrication. Asillustrated in FIG. 15, the vectors continuously change directions,which is non-ideal for fabrication.

To enhance fabrication efficiency, the direction at which the programshould search for matching coordinates is prioritized, as illustrated inFIG. 16. The program is first instructed to search in the +x direction(FIG. 16 a). The software compares the starting segment to the adjacentone directly to the right, and if the segments match, a new vector pathis created. This process continues until no more matching index can befound along the +x direction. The program then searches in the seconddirection (−y direction; FIG. 16 b), and vectors paths are created inthe −y direction if matching segments are found. If there is no match,the software moves onto the next direction (−x direction; FIG. 16 c).Once all direction has been explored, the newly formed vectors alongsame directions are added to form continuous vector paths (FIG. 16 d). Anew segment that has not been processed is then selected as a startingpoint, and the comparison process repeats until all segments have beenprocessed.

Specifying and Fabricating Geometries Exhibiting AnisotropicHeterogeneity Based on Mathematical Models

In contrast to the process for specifying and fabricating geometriesexhibiting anisotropic heterogeneity based on prescribed imagesdescribed herein above, in this aspect printable models are directlycreated (i.e., not from prescribed images). In addition, this processaspect enables control of the gradient establishment. Once the printablegeometry is dithered, however, the vector conversion process remainssimilar to the aforementioned process wherein the target object isobtained from a prescribed image.

FIG. 17 is a flowchart 3000 describing the programmable machine-enabledprocess steps for specifying (and ultimately fabricating) geometriesexhibiting anisotropic heterogeneity based on mathematical models, thatare different than the process steps for specifying and fabricatinggeometries exhibiting anisotropic heterogeneity based on prescribedimages described herein above.

The steps include defining the 3-D geometry of a selected target object;determining a gradient distribution of material compositions of thetarget object to be applied to the geometry; establishing a probabilitydistribution function based on the gradient distribution to determine aplurality of path segments and assigning a plurality of one or more ofthe path segments to a corresponding plurality of materials from whichthe target object will be fabricated; dithering the pluralities of pathsegments based on the probability distribution function; converting thedithered path segments into respective sets of vector pathscorresponding to respective ones of the materials, wherein each set ofvector paths is one or more line elements of a given length andorientation; and creating a vector print file for the target objectreadable by a compatible fabrication apparatus.

FIG. 18 illustrates a printable layer exhibiting a locally heterogeneouslinear gradient based on the one-dimensional probability equations shownin the figure, where p represents the probability of the segment beingassigned to material 1, x:—is the x-coordinate on the target segment, iis the layer number (1 is the bottom layer), n is the number of layers.

FIGS. 19 and 20, respectively, illustrate printable patterns exhibitinglocally heterogeneous diagonal, and circumferential gradients based onthe illustrated probability equations. (p: probability of segment beingassigned to material 1, x: x-coordinate on target segment, y:y-coordinate on target segment, r: radius of target circle, i: layer #(1 is the bottom layer), n: # of layers). Note that gradientdistribution need not be prescribed by a single equation, or evenequations for the whole 3D material space.

FIG. 21 shows an example of a locally heterogeneous geometry printedwith multiple hydrogel blends.

Based upon the foregoing specification (and fabrication) method aspects,it will be appreciated that implementing a similar vector path algorithmto separately control an on-board photocrosslinking unit simultaneouslywith printing will provide unique crosslinking behavior not mimicked bysimply locating a UV lamp (or other suitable energy source) near thecompleted deposition, or even locating the lamp proximate to thematerial while it is being deposited. In each of the latter cases,spatial and temporal control of crosslinking is extremely limited. Theembodied implementation will enable controlled photo-curing, allowingthe capability of seamless joining of consecutive deposited layers andgradient material distribution between deposited lines. This willprovide the ability to control when and where to crosslink, furtherenabling layer and/or line specific blending with a continuous materialgradient between them via diffusion of the initially liquid likeelements.

It is to be especially noted that the embodied object specification andfabrication method embodiments do not apply solely to a particular typeof medium, method of material deposition, or method of solidification(e.g., curing). Rather, the invention pertains to specifying andgenerating vector based deposition paths for a selected material(s) tospecify and, optionally fabricate an object of interest. Althoughextruding the material(s) is disclosed as an advantageous aspect of theinvention, other techniques may include sintering material(s) in thesepaths, laying molten metal in these paths, etc. Likewise, two or morematerials need not be able to diffuse into each other. This all dependson the user's wishes. While we have demonstrated vector-based traversalof a UV LED array for photocrosslinking various hydrogel materials, theinvention is drawn more generally to any applicable chemical or otherenergy source useful for solidifying or structurally stabilizing amaterial(s) during object fabrication. The disclosed and claimed processembodiments and aspects are intended to enable any and all of thesevariations and options.

Glossary of Terms

Computer Aided Design (CAD)—use of computers to draft and/or generatefigures in two-dimensional space and solids in 3-dimensional space.

Chemical crosslinking—a chemical catalyses or initiates a reaction thatbonds polymer chains together over time. In contrast tophoto-crosslinking method, chemical crosslinking is time-dependent.

-   -   Example 1: polymer=alginate, crosslinker=CaCl₂

-   -   Binding of Ca₂+ to -L-GulUA-(1-4)-L-GulUA unit in alginate [20]        [please cite]. The main attractive interaction is of        anelectrostatic nature represented by an attraction between the        cation and partial negatively charged oxygen atoms structures        when they react with multivalent ions.    -   Example 2: Silicone Sealant—Room temperature vulcanizing        silicone sealant is cured by moisture in the air.

Computed Tomography (CT) scan—medical imaging technology using X-rays toimage internal structures in 3-dimensional space. The densityinformation from an X-ray CT scan is typically in Hounsfield units. Thedensity data from a CT scan can be communicated in a DICOM file format.

Crosslinking—bonds that link one polymer chain to another. They can becovalent or ionic bonds.

Digital Imaging and Communications in Medicine (DICOM)—standard imageformat for handling, storing, printing and transmitting informationstored in medical images, such as CT and MRI scans. It includes a fileformat definition and a network communications protocol.

Gel—a term associated with highly swollen polymer networks. A geltypically consists of a cross-linked polymer network holding a largeamount of liquid.

Heterogeneous (Merriam-Webster)—adj. consisting of dissimilar or diverseingredients or constituents

Hounsfield (HU) scale—scale for describing radiodensity. Calibrated withreference to water. Intensity in an image indicates radiodensity. Boneshave ˜700 to 3000 HU, while soft tissues such as heart valves can rangefrom −300 to −100 HU. Air is −1000 HU, and water is 0 HU.

Hydrogel—consists of water soluble polymers and holds water atequilibrium.

Magnetic Resonance Imaging (MRI)—medical imaging technique used tovisualize internal structures using nuclear magnetic resonance (NMR).Can provide improved contrast between different soft tissues, such asmuscles and heart.

Mimics®—commercially available software designed to process DICOMsobtained from CT and MRI scans and reconstruct the scan into CADgeometries.

Nuclear Magnetic Resonance (NMR)—phenomenon where nuclei exposed tomagnetic field absorb and emit electromagnetic radiation.

Poly(ethylene) diacrylate (PEG-DA)—a photocrosslinkable polymer that candissolve in water. Dissolving photoinitiators into PEG-DA precursorsolution and exposing the solution to UV light can polymerize theprecursor solution into a hydrogel.

Photo-initiator—a chemical compound that decomposes into free radicalswhen exposed to light. The free-radicals react with polymer chains thathave available double bonds or functional groups to initiatephoto-polymerization.

Photo-crosslinking (or photo-polymerization)—use of light source withspecific energy to trigger reactions that bonds polymer chains together

Polymerization—process of joining small molecules by covalent bonds toproduce high-molecular weight polymers.

-   -   Example: Polymer=PEG-DA, crosslinker=Photo-initiator=Irgacure.        PEG-DA has two functional end groups available for crosslinking.        With a photo-initiator in the polymer solution, exposing the        solution to UV light will create free radicals which will react        with the functional ends of the PEGDA chains.

Solid Free-Form Fabrication or 3-D printing—general term for additivemanufacturing. The Fab @Home™ printer we have been using falls under thecategory of Robocasting. Viscous liquid materials are extruded to buildthree dimensional shapes layer by layer.

Standard Tesselation Language (STL)—file format used instereolithography CAD software, rapid prototyping software, and CADmanufacturing. STL geometries describes the surface geometry of a threedimensional object without any internal structure information—it isessentially the shell of a 3D shape. The Fab @Home™ 3-D printer softwarecan read a STL geometry, slice it into layers, and plot print paths forfabricating the geometry in 3-D.

Vector—a set of coordinate points to define an extrusion path.

All references, including publications, patent applications, and patentscited herein are hereby incorporated by reference in their entireties tothe same extent as if each reference was individually and specificallyindicated to be incorporated by reference and was set forth in itsentirety herein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. The term “connected” is to beconstrued as partly or wholly contained within, attached to, or joinedtogether, even if there is something intervening.

The recitation of ranges of values herein is merely intended to serve asa shorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wasindividually recited herein.

All methods described herein can be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contradicted bycontext. The use of any and all examples, or exemplary language (e.g.,“such as”) provided herein, is intended merely to better illuminateembodiments of the invention and does not impose a limitation on thescope of the invention unless otherwise claimed.

No language in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. There isno intention to limit the invention to the specific form or formsdisclosed, but on the contrary, the intention is to cover allmodifications, alternative constructions, and equivalents falling withinthe spirit and scope of the invention, as defined in the appendedclaims. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

Therefore, the embodiments are illustrative of the invention rather thanlimiting of the invention. Revisions and modifications may be made tomethods, materials structures and dimensions in accordance with thedisclosed embodiments, further in accordance with the accompanyingclaims.

We claim:
 1. A machine-implemented method for specifying a target objectto be fabricated, comprising the steps of: i) defining the target objectto be fabricated; ii) determining a locally-heterogeneous, anisotropicdistribution of phenotypic characteristics of the target object to befabricated; iii) ascribing the distribution of phenotypiccharacteristics to a plurality of materials from which the target objectwill be fabricated; iv) stitching together each distributioncorresponding to respective ones of the materials into respective setsof vector paths corresponding to respective ones of the materials,wherein each set of vector paths is one or more line elements of a givenlength and orientation; and v) creating a vector print file for thetarget object readable by a compatible fabrication apparatus.
 2. Themethod of claim 1, wherein the step of defining the target object to befabricated comprises one of sourcing the target object from a prescribedimage and sourcing the target object from a mathematical model.
 3. Themethod of claim 2, wherein the step of sourcing the target object from aprescribed image further comprises reconstructing the image of thetarget object into a 3-D machine-readable format.
 4. The method of claim3, further comprising: obtaining a plurality of image layers;determining a desired image resolution; selecting one or more of theplurality of image layers as an n^(th) fabrication layer image, where nis equal to or greater than one; and optionally, resizing the n^(th)fabrication layer image for specifying at least one of a size and avolume characteristic of the target object.
 5. The method of claim 4,wherein the step of determining a locally-heterogeneous, anisotropicdistribution of phenotypic characteristics of the target object to befabricated further comprises assigning values between zero to one to aeach of a plurality of pixels or voxels, or to each of a plurality ofsets of adjoining pixels or voxels, of the n^(th) fabrication layerimage.
 6. The method of claim 5, wherein the step of ascribing thedistribution of phenotypic characteristics to a plurality of materialsfrom which the target object will be fabricated further comprisesassociating a plurality of the assigned values to respective ones of thematerials.
 7. The method of claim 6, wherein the step of associating aplurality of the assigned values to respective ones of the materialsfurther comprises thresholding the assigned values.
 8. The method ofclaim 6, wherein the step of associating a plurality of the assignedvalues to respective ones of the materials further comprises determiningHounsfield unit values for the pixels or voxels.
 9. The method of claim6, further comprising removing background that is not a part of thetarget object from the n^(th) fabrication layer image.
 10. The method ofclaim 6, wherein the step of stitching together each distributionfurther comprises dithering each of the plurality of the assigned valuesinto a plurality of segments corresponding to respective ones of thematerials.
 11. The method of claim 10, further comprising utilizing aFloyd-Steinberg dithering algorithm.
 12. The method of claim 10, furthercomprising converting each of the plurality of segments into the vectorpaths for the respective ones of the materials.
 13. The method of claim12, wherein the step of creating a vector path further comprises: a)selecting a starting index; b) comparing the starting index to anadjacent index in a specified direction; c) determining whether thestarting and adjacent indices are assigned to the same one of thematerials, and d) if the starting and adjacent indices are assigned tothe same one of the materials, then (e) creating a vector path betweenthe indices, or f) if the starting and adjacent indices are not assignedto the same one of the materials, then g) changing the adjacent indexinto a new starting index and performing steps (b-e); and h) performingsteps (a-g) until a sufficient number of vector paths have beengenerated to specify the target object.
 14. The method of claim 1,further comprising iteratively performing steps (iii-v) for a pluralityof n fabrication layers, where n is equal to or greater than one. 15.The method of claim 13, further comprising iteratively performing steps(iii-v) for a plurality of the n^(th) fabrication layers, where n isequal to or greater than one.
 16. The method of claim 1, wherein thephenotypic characteristics of the target object to be fabricated aresurface and/or internal phenotypic characteristics.
 17. The method ofclaim 1, further comprising outputting the vector print file for thetarget object to the compatible fabrication apparatus and fabricatingthe target object.
 18. The method of claim 17, further comprisingfabricating the target object in a series of n fabrication layers, andphoto-curing a selected region of each n^(th) fabrication layersimultaneously with the layer fabrication or prior to the fabrication ofthe n^(th)+1 layer.
 19. The method of claim 15, further comprisingoutputting the vector print file for the target object to the compatiblefabrication apparatus and fabricating the target object.
 20. The methodof claim 19, further comprising fabricating the target object in aseries of n fabrication layers, and photo-curing a selected region ofeach n^(th) fabrication layer simultaneously with the layer fabricationor prior to the fabrication of the n^(th)+1 layer.
 21. The method ofclaim 2, wherein the step of sourcing the target object from amathematical model further comprises: defining the 3-D geometry of aselected target object; determining a gradient distribution of materialcompositions of the target object to be applied to the geometry;establishing a probability distribution function based on the gradientdistribution to determine a plurality of path segments and assigning aplurality of one or more of the path segments to a correspondingplurality of materials from which the target object will be fabricated;dithering the pluralities of path segments based on the probabilitydistribution function; converting the dithered path segments intorespective sets of vector paths corresponding to respective ones of thematerials, wherein each set of vector paths is one or more line elementsof a given length and orientation; and v) creating a vector print filefor the target object readable by a compatible fabrication apparatus.22. The method of claim 21, wherein the gradient distribution is linear.23. The method of claim 21, wherein the gradient distribution isnon-linear.
 24. The method of claim 21, wherein the gradientdistribution is radial.
 25. The method of claim 21, further comprisingoutputting the vector print file for the target object to the compatiblefabrication apparatus and fabricating the target object.
 26. The methodof claim 25, further comprising fabricating the target object in aseries of n fabrication layers, and photo-curing a selected region ofeach n^(th) fabrication layer simultaneously with the layer fabricationor prior to the fabrication of the n^(th)+1 layer.